MAX1887EEE Maxim Integrated Products, MAX1887EEE Datasheet - Page 24

• Manufacturer Part Number: MAX1887EEE
• Description: Current Mode PWM Controllers
• Manufacturer: Maxim Integrated Products

Specifications of MAX1887EEE

• Number Of Outputs: 1
• Mounting Style: SMD/SMT
• Package / Case: QSOP-16
• Switching Frequency: 550 KHz
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Synchronous Pin: No
• Topology: Boost

The input capacitor must meet the ripple current
requirement (I
The MAX1887/MAX1897 multiphase slave controllers
operate out-of-phase (MAX1897 POL = V
staggering the turn-on times of each phase. This mini-
mizes the input ripple current by dividing the load cur-
rent among independent phases:
for out-of-phase operation.
When operating the MAX1897 in-phase (POL = GND),
the high-side MOSFETs turn on simultaneously, so
input capacitors must support the combined input rip-
ple currents of each phase:
for in-phase operation.
For most applications, nontantalum chemistries (ceram-
ic, aluminum, or OS-CON) are preferred because of
their resistance to inrush surge currents typical of sys-
tems with a mechanical switch or connector in series
with the input. If the MAX1887/MAX1897 is operated as
the second stage of a two-stage power-conversion sys-
tem, tantalum input capacitors are acceptable. In either
configuration, choose an input capacitor that exhibits
less than +10°C temperature rise at the RMS input cur-
rent for optimal circuit longevity.
Most of the following MOSFET guidelines focus on the
challenge of obtaining high load-current capability
when using high-voltage (>20V) AC adapters. Low-cur-
rent applications usually require less attention.
The high-side MOSFET (N
the resistive losses plus the switching losses at both
V
Ideally, the losses at V
to losses at V
the losses at V
losses at V
Conversely, if the losses at V
higher than the losses at V
the size of N
Quick-PWM Slave Controllers for
Multiphase, Step-Down Supplies
24
IN(MIN)
______________________________________________________________________________________
I
and V
RMS
I
RMS
IN(MAX)
H
=
RMS
IN(MAX)
. If V
=
IN(MAX)
IN(MIN)



I
LOAD
I
LOAD
) imposed by the switching currents.
, consider increasing the size of N
IN
η
, with lower losses in between. If
does not vary over a wide range,




Input Capacitor Selection
Power MOSFET Selection
. Calculate both of these sums.



IN(MIN)
are significantly higher than the




V
OUT IN
H
V
) must be able to dissipate
IN(MIN)
OUT IN
(
should be roughly equal
IN(MAX)
V
V
IN
(
V
V
IN
−
, consider reducing
V
−
OUT
V
are significantly
OUT
)




CC
)




or float),
H
.
the minimum power dissipation occurs where the resis-
tive losses equal the switching losses.
Choose a low-side MOSFET that has the lowest possi-
ble on-resistance (R
sized package (i.e., one or two SO-8s, DPAK or
D
DL gate driver can supply sufficient current to support
the gate charge and the current injected into the para-
sitic gate-to-drain capacitor caused by the high-side
MOSFET turning on; otherwise, cross-conduction prob-
lems may occur.
Worst-case conduction losses occur at the duty factor
extremes. For the high-side MOSFET (N
case power dissipation due to resistance occurs at the
minimum input voltage:
Generally, a small high-side MOSFET is desired to
reduce switching losses at high input voltages.
However, the R
power-dissipation often limits how small the MOSFET
can be. Again, the optimum occurs when the switching
losses equal the conduction (R
side switching losses don’t usually become an issue
until the input is greater than approximately 15V.
Calculating the power dissipation of the high-side
MOSFET (N
must allow for difficult quantifying factors that influence
the turn-on and turn-off times. These factors include the
internal gate resistance, gate charge, threshold volt-
age, source inductance, and PC board layout charac-
teristics. The following switching-loss calculation
provides only a very rough estimate and is no substi-
tute for breadboard evaluation, preferably including
verification using a thermocouple mounted on N
where C
and I
(1A typ).
Switching losses in the high-side MOSFET can become
an insidious heat problem when maximum AC adapter
voltages are applied, due to the squared term in the C
✕
MOSFET chosen for adequate R
voltages becomes extraordinarily hot when biased from
2
PAK), and is reasonably priced. Make sure that the
V
IN
PD N
PD N Switching
GATE
2
✕
(
(
RSS
f SW
H
H
Re
is the peak gate-drive source/sink current
H
) due to switching losses is difficult since it
is the reverse transfer capacitance of N
switching-loss equation. If the high-side
sistive
DS(ON)
MOSFET Power Dissipation
)
=
DS(ON)
)
=
required to stay within package



V
(
V
OUT
V
IN MAX
IN
(
), comes in a moderate-






I
DS(ON)
LOAD
)
DS(ON)
)
I
2
η
GATE
C
RSS SW LOAD



) losses. High-
2
η
at low battery
H
R
f
), the worst-
DS ON
I
(
H
:
)
H